Genetic drift in finite populations

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Genetic drift in finite populations HWE
assumes that population size is
infinite approximately true if population is
very large
but, very large size unlikely for most
species -- environmental patchiness -- social
organization
evolutionarily relevant population size is not
simply the number of individuals
Ne effective population size number of
individualsthat contribute genes to the next
generation
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Effective population size, Ne affected by --
sex ratio (breeding system) -- fluctuations in
population size -- differences in fecundity due
to chance
Mating systems where Sex Ratio greatly skewed
alter Ne -- dominance hierarchy --
resource polygyny -- leks
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Ne and population fluctuations if
population size fluctuates frequently, the
arithmetic mean is not appropriate
hare-lynx cycles
humans in Egypt
Ne 100 150 25 150 125 Ne 110
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average Ne affected disproportionately by
the smallest size --gt population
bottleneck results in the loss of genetic
variation
With a fluctuating population use the
harmonic mean (geometric average)
Ne 100 150 25 150 125 Ne 70
population fluctuations characterize irruptive
species -- forest insects (e.g., gypsy
moth) -- seed-eating finches -- voles hare
lynx -- coniferous trees
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if Ne is small, what are the genetic
consequences?? -- neutral alleles --
interaction of selection and small population
size
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genetic drift is sampling effect true
frequency distn vs. sample frequency
distn take finite samples from an infinite
population ---gt most will deviate slightly
from the actual distribution (frequency of A1,
A2)
if you have a large number of samples, the
average frequency over all samples will better
approximate the true distribution than any single
sample infinite number of gametes ---gt
finite number of adults (population) (sample
)
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true mean 0.35
X 0.361 0.105 X 0.345 0.032
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Random walk of allele frequency in one
dimension - direction of change is random in
each generation - amount of change is
determined by Ne (Dq )

1 Ne
0 q0 1
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2N 18
2N 100
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What is the probability that an allele is fixed
by drift??
If A1, A2, A3, A4, , An alleles in a
population N individuals, 2N alleles -- one
is fixed, others lost
If each allele is unique, pr(Ai) is fixed
1/2N (2N possible outcomes, pr(particular)
1/2N)
With x copies of an allele in the
population, pr(fixation) x/2N common
alleles are more likely to be fixed
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how does drift affect heterozygosity?? because
alleles are fixed or lost, heterozygosity and
within popn genetic variance declines
What is the chance that two identical alleles
from an individual unite to form a zygote?
1/2N
When they do heterozygosity is lost. What is the
chance that they do not unite?
1- 1/2N
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how does drift affect heterozygosity?? because
alleles are fixed or lost, heterozygosity and
within popn genetic s2 decline After t
generations Ht (1 - )tH0 if 2N is
large, 1/2N 0, and (1 - ) 1, Ht
H0 if 2N is small, 1/2N , and
(1 - ) 0, Ht 0
1 2N
1 2N
1 2N
very large
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drift changes allele frequencies within a
population in a random fashion
drift decreases within population genetic variance
net change in mean allele frequency (across all
popns) due to drift is 0
drift increases among population genetic variance
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Drift in real populations Buri Drosophila
melanogaster alleles at brown eye color
locus bw, bw75 are neutral to each
other initially f(bw) f(bw75) 0.5 107
replicate lines, Ne 16 (8, 8), each
generation randomly chose 16 new
individuals what happens to genetic variation???
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Buris results by generation 19, 30 popns fix
bw 28 popns fix bw75 which allele fixed is
random decrease in within popn variance
(58/107 fixed for one allele) increase in
among population genetic variance (s2 0
---gt 0.16)
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Genetic drift and selection Drift fixes alleles
(at random) when Ne is small Directional
selection fixes the favored allele When does
drift overcome selection (or vice versa)??
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interaction of drift and directional
selection AA Aa aa initial f(A) 0.5 wij
0.4 1 1 N50 N10
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interaction of drift and directional
selection AA Aa aa initial f(A) 0.5 wij
0.8 1 1 N10
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interaction of drift and selection AA Aa a
a initial f(A) 0.5 wij 0.8 1 0.9
N50
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Genetic drift and selection Drift fixes alleles
(at random) when Ne is small Directional
selection fixes the favored allele When does
drift overcome selection ( vice versa)?? s
strength of selection strength of
genetic drift if s gtgt 1/4Ne, selection
determines outcome if s ltlt 1/4Ne, drift can
overcome selection
1 4Ne
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s 0.01 Ne 10 1/4Ne 0.025 25
0.01 100 0.0025 250
0.001
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Drift and Selection, cont Population may not
attain equilibrium allele frequencies
predicted by selection coefficients Deleterious
alleles can be fixed by drift if selection
is relatively weak Slightly advantageous
alleles are less likely to be fixed in small
populations
Implications for conservation biology -
habitat fragmentation ---gt small population
size - loss of heterozygosity - fixation of
deleterious alleles ---gt mutational meltdown
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Founder Event Genetic drift occurs when a new
population is started by a small number of
individuals Usually, loss of rare or uncommon
alleles (i.e., reduced polymorphism, within
popn genetic s2)
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Founder event Genetic drift occurs when a new
population is started by a small number of
individuals Usually, loss of rare or uncommon
alleles (i.e., reduced polymorphism, within
popn genetic s2) Loss of variation may
differentially affect social organisms
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Ne, the effective population size, can be
affected by mating system, temporal fluctuations
in population size, or differences in fecundity
due to chance Genetic drift is sampling
error Drift occurs in all finite populations,
but its relative importance is determined by
population size Genetic drift reduces genetic
variation within a population by random fixation
of alleles, but increases genetic variance among
populations Directional selection in small
populations is often less effective deleterious
alleles may occur at higher frequency and
may become fixed
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Genome-wide effects of drift may result in a high
genetic load of deleterious alleles and
consequent effect on individual and population
survival (mutational meltdown) Founder events
can produce new populations with radically
different patterns of genetic variation compared
to their source - loss of alleles, especially
rare alleles - changes in allele frequency -
changes in genetic variance depend on
subsequent rates of growth